1. Field of the Invention
The present invention concerns a production method for carbon coated optical fibers. In particular, the present invention concerns a production method for carbon coated optical fibers demonstrating improved coating deposition rate.
2. Prior Art
Up to now, quartz optical fibers have been used for communication cables and the like. However, when these quartz optical fibers come into contact with hydrogen, the hydrogen diffuses into the fiber, and due to molecular vibration of the hydrogen molecules, there is the problem of increased absorption losses. Also, dopants contained in the cables, such as P.sub.2 O.sub.5, GeO.sub.2, and B.sub.2 O.sub.3 and the like react with hydrogen that has diffused into the cable, thereby forming OH radicales that result in transmission losses. In order to remedy these problems, a method of charging the interior of these cables with hydrogen absorbing liquids (Japanese Patent Publication Application Kokai No. 61-251808) and other methods have been considered. However, with these methods, not only have the results been inadequate, but the processes are complicated and, practically speaking, not suitable.
In answer to this, recently Corning Glass (International Wire & Cable Symposium Proceedings 1987, pages 241-244, and Journal of Lightwave Technology, Vol. 6, No. 2, February 1988, pages 240-244) and AT&T (Electronic Letters, 13th October 1988 Vol. 24, No. 21, pages 1323-1324, and OFC '88/Tuesday Afternoon/23) have published a chemical vapor deposition method (CVD method) in which a carbon coating is formed on the optical fiber surface in order to improve its hydrogen resistance. With this production method, in a spinning furnace, as the bare uncoated optical fiber is fed into the thermal-CVD furnace, a hydrocarbon compound is thermally decomposed, thereby forming a carbon coating on the bare optical fiber surface. However, with such a process, as the fiber is heated in order to form the carbon coating, water molecules absorbed on the fiber surface form silanol groups. These silanol groups erode the optical fiber surface and cause minute cracks which tend to extend leading to the problem of deterioration in the mechanical strength of the fiber.
Furthermore, in addition to the previously mentioned water, nitrogen, oxygen, and the like are absorbed on the optical fiber surface which not only causes a deterioration in the coating deposition rate, but also causes the problem of diminished adherence of the coating to the underlying optical fiber.
Moreover, in the above described process, the entire CVD furnace used to deposit the carbon coating on the bare fiber must be maintained at the decomposition temperature of the starting material. Therefore, in order to produce a coated optical fiber of which the coating layer is sufficiently thick to provide suitable mechanical strength and hydrogen resistance properties, the spinning rate must be controlled at on the order of from 10 to 20 m/min which causes the additional problem of inability to achieve high speed spinning.